diff --git a/stix2/test/v20/test_custom.py b/stix2/test/v20/test_custom.py
index 5a29873..e4b5377 100644
--- a/stix2/test/v20/test_custom.py
+++ b/stix2/test/v20/test_custom.py
@@ -168,7 +168,7 @@ def test_custom_property_in_observed_data():
         allow_custom=True,
         first_observed="2015-12-21T19:00:00Z",
         last_observed="2015-12-21T19:00:00Z",
-        number_observed=0,
+        number_observed=1,
         objects={"0": artifact},
     )
 
@@ -190,7 +190,7 @@ def test_custom_property_object_in_observable_extension():
         allow_custom=True,
         first_observed="2015-12-21T19:00:00Z",
         last_observed="2015-12-21T19:00:00Z",
-        number_observed=0,
+        number_observed=1,
         objects={"0": artifact},
     )
 
@@ -225,7 +225,7 @@ def test_custom_property_dict_in_observable_extension():
         allow_custom=True,
         first_observed="2015-12-21T19:00:00Z",
         last_observed="2015-12-21T19:00:00Z",
-        number_observed=0,
+        number_observed=1,
         objects={"0": artifact},
     )
 
diff --git a/stix2/v20/sdo.py b/stix2/v20/sdo.py
index 40c987e..3fcfed0 100644
--- a/stix2/v20/sdo.py
+++ b/stix2/v20/sdo.py
@@ -203,7 +203,7 @@ class ObservedData(STIXDomainObject):
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('first_observed', TimestampProperty(required=True)),
         ('last_observed', TimestampProperty(required=True)),
-        ('number_observed', IntegerProperty(required=True)),
+        ('number_observed', IntegerProperty(min=1, max=999999999, required=True)),
         ('objects', ObservableProperty(required=True)),
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
diff --git a/stix2/v20/sro.py b/stix2/v20/sro.py
index 9abb38a..dbf6812 100644
--- a/stix2/v20/sro.py
+++ b/stix2/v20/sro.py
@@ -64,7 +64,7 @@ class Sighting(STIXRelationshipObject):
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('first_seen', TimestampProperty()),
         ('last_seen', TimestampProperty()),
-        ('count', IntegerProperty()),
+        ('count', IntegerProperty(min=0, max=999999999)),
         ('sighting_of_ref', ReferenceProperty(required=True)),
         ('observed_data_refs', ListProperty(ReferenceProperty(type='observed-data'))),
         ('where_sighted_refs', ListProperty(ReferenceProperty(type='identity'))),